1. Field of the Invention
The present invention relates to a manufacturing method of a gear device for use in an electric power steering apparatus for a vehicle or the like.
2. Description of Related Art
An electric power steering apparatus for a vehicle includes an electric motor for assisting steering and a gear device which transmits a rotational force of the electric motor to steering means, and assists movement of the steering means corresponding to operation of a steering wheel by rotation of the electric motor, thereby reducing a driver's labor and burden for steering.
The gear device includes a worm interlockingly coupled to an output shaft of the electric motor via a shaft joint, a worm wheel engaged with the worm and connected to the steering means, and a housing for rotatably supporting and accommodating the worm and the worm wheel (for example, Japanese Patent Application Laid-Open No. 2002-21943).
The worm, having helical teeth integrally formed on an outer peripheral part of a right circular cylinder member made of metal material, is disposed so as to be intersected with the axis core of the worm wheel and is supported in the housing via a pair of rolling bearings.
The worm wheel is engaged with and fixed into a rotational shaft of the steering means and the rotational shaft is supported in the housing via the pair of rolling bearings.
The housing has a first supporting section into which the rolling bearing is fitted and in which the worm is rotatably supported, and a second supporting section into which the rolling bearing is fitted and in which the worm wheel is rotatably supported.
The worm and worm wheel of the gear device are combined so that any movement in each radial direction is not permitted; however, since there are dimensional errors in each processed worm, worm wheel, and housing, a backlash amount at the engagement part between the worm and the worm wheel varies from small to large when they are combined neglecting their dimensional errors. When the backlash amount is large, rattling noise is generated when steering, and the rattling noise leaks into an automobile compartment. Further, torque increases when the backlash amount is small, thus the worm and worm wheel cannot be smoothly rotated.
FIG. 1 is an explanatory view showing a layered assembly process of the gear device. Conventionally in order to have a proper backlash amount and torque in engagement parts between articles, processed worm wheel 100, worm 101, housing 102, and the like are classified into a plurality of dimension groups with respect to design dimensions so that a distance between the centers of the worm wheel 100 and worm 101 is set within an allowable range by selectively combining one set of the worm wheel 100, worm 101, and housing 102 from the thus-classified dimension groups (a so called layered assembly).
However, in the conventional gear devices, the worm wheel 100, worm 101, and housing 102 are individually measured and the layered assembly is then performed; and therefore, measurement errors generated at each measurement are accumulated when assembling; further, measurement errors due to the atmospheric temperature also largely affects. Consequently, there has been a problem in that the backlash amount and torque are not stable between articles. Further, stocks of products in process for layered assembly increase and the increased stocks cause high cost, and therefore, countermeasures are required.